1. Field of the Invention
The present invention relates to a method of treating gold plating film, and more specifically to a method of treating a gold layer plated on electric contact material for instance, so that the thickness of the gold plating layer can be reduced without deteriorating the contact resistance and durability thereof.
2. Description of the Prior Art
Electric contacts are widely used in various industrial fields, and the surface of the electric contact is usually plated with a noble metal, in particular with gold to improve the surface stability and durability of the contact, because gold is low in contact resistance and high in corrosion resistance.
In almost all the gold plating film, however, since there are minute defects such as pores, where the film thickness is thin, the upper surface of the gold film communicates with the lower surface thereof (referred to as pinholes). Therefore, when the gold film is exposed within a corrosive environment, there exists a problem in that a local battery corrosion phenomenon is produced between a substrate metal (nickel plating layer, in usual) and the gold plating layer, so that there exists a phenomenon such that corrosion product or substance of the substrate is precipitated from the surface thereof, thus resulting in an increase in the contact resistance of the electric contact. Conventionally, therefore, it has been necessary to cover the surface of the contact with a relatively-thick gold plating film, in order to eliminate the minute pores within the film as small as possible or to prevent the gold film deterioration.
Experiments have indicated that a sufficient thickness of the gold film is about 0.5 .mu.m when only the contact resistance and abrasion resistance are taken into account. In practice, however, since the thickness of gold plating film lies from 2.0 to 2.5 .mu.m under due consideration of the above-mentioned local battery corrosion caused by the presence of pores, there exists a problem in that the cost of the contact material inevitably increases.
In addition, where there exist a great number of lattice defects in the gold plating film, since corrosion grows beginning from these lattice defects, the durability of the contact parts is further deteriorated by the lattice defects.